Measuring the mass and spin of new particles using $M_{T2}$ variable at the LHC

Abstract

It is expected that various new physics beyond the Standard Model (SM) inevitably lead to the new events with 2 lightest stable neutral particles(LSNP) escaping the detector of the LHC. Due to the existence of the 2 LSNPs, the new physics events are hard to be reconstructed and some nontrivial methods are required for measuring the mass and spin of new particles. $M_{T2}$ is the extension of the ordinary transverse mass for the events with 2 missing particles. Using the $M_{T2}$ variable in the symmetric event with 2 LSNPs, we found that the new particle masses can be constrained by the analytic expression of $M_{T2}^{max} \chi$, which is the maximum value of the $M_{T2}$ with trial LSNP mass, $\chi$. Furthermore, the $M_{T2}^{max} \chi$ has kink structure at the correct point of mother particle and LSNP masses, which is originated from various extreme momentum configurations with different functional behavior of $M_{T2} \chi$ with respect to the trial LSNP mass. Then, the mother particle and LSNP masses can be determined simultaneously by identifying the kink position. For spin measurement, we also found that the $M_{T2}$-Assisted On-Shell (MAOS) reconstruction could help to resolve the spin correlation structure of new event with 2 LSNPs, providing the additional angular correlation information involving the LSNP, which have been considered to be mostly impossible. In this thesis we will describe all of these features of measuring new particle masses and spins using the $M_{T2}$ variable.